electrical engineer student, needs help!

[pbandj]

Hello I'm a electrical engineer student at my local community college. The class I'm in right now is introduction to DC circuit. Well not to sound completely ignorant but I have no clue whats going on in the class. The teacher is one of those reads out of the books no questions asked teacher. So he's utterly useless. Anyway I was wondering if someone here could help me.

Here's my questions and confusions:

First thing I'm having issues with is resistors. On the homework he gives us the colors and I have to figure out the value. Not to complicated until I got to this one. Orange, White, Yellow. Orange is 3 white is 9 and yellow is 4 so that means its 390000 ohms so how would you write that 3.9K ohms or 390k ohms?

Another issue I'm having is computing the potential difference between two points. On the HW he has a resistor point a on one end and point b on the other end. Point A is 35V and point B is 15V my only question is do you add them to figure out potential difference? Or is there something else you have to do?

This might be hard to explain without showing you so I'm going to quote the exact question word for word. “Write in scientific notation the charge carried by 100 electrons.” I honestly don't know where to begin with that.

Thanks for the help.

 

[MikeMl]

...

First thing I'm having issues with is resistors. On the homework he gives us the colors and I have to figure out the value. Not to complicated until I got to this one. Orange, White, Yellow. Orange is 3 white is 9 and yellow is 4 so that means its 390000 ohms so how would you write that 3.9K ohms or 390k ohms?

Can be written as 390K or 390000Ω. 3.9K would be 3900Ω. On a three band resistor, the last color is the power of ten , i.e. yellow is a multiplier of 10^4 = 10000

Another issue I'm having is computing the potential difference between two points. On the HW he has a resistor point a on one end and point b on the other end. Point A is 35V and point B is 15V my only question is do you add them to figure out potential difference? Or is there something else you have to do?

The "drop" across the resistor from A to B is written as V(A) - V(B) = 35 - 15 = 20V.

Suppose point A is at +35V and Point B is at -15V, then 35 - (-15) would be a drop of 50V.

Further suppose that point A is at -35V and point B is at +15V, then -35 - 15 would be a drop of -50V, showing that a negative drop is allowed, but then it wouldn't be a resistor, it would be a voltage source, instead.

This might be hard to explain without showing you so I'm going to quote the exact question word for word. “Write in scientific notation the charge carried by 100 electrons.” I honestly don't know where to begin with that...

First you have to know the charge carried by one electron. Wiki says it is −1.602176487×10−19 Coulombs. I'll leave it up to you to do the math.

 

[pbandj]

thanks that really helped me out a lot

 

[cr0sh]

Something else to help you out: Color Code Mneumonic (kinda dirty, too):

Bad Boys Rape Our Young Girls, But Violet Gives Willingly

 

[DerStrom8]

Something else to help you out: Color Code Mneumonic (kinda dirty, too):

Bad Boys Rape Our Young Girls, But Violet Gives Willingly

There's also one that isn't quite as "dirty" :
Billy Bob Runs Out Yonder, Giving Babies Very Greasy Wigs (Black Brown Red Orange Yellow Green Blue Violet Gray White)
It sounds kind of stupid, but it's how I learned
Der Strom

 

[cr0sh]

There's also one that isn't quite as "dirty" :
Billy Bob Runs Out Yonder, Giving Babies Very Greasy Wigs (Black Brown Red Orange Yellow Green Blue Violet Gray White)
It sounds kind of stupid, but it's how I learned
Der Strom

That's a different one...

When I was taught the one I know, the instructor also passed along a less risqué form - but I don't remember it! I think it's mainly because at the time I was 18, fresh out of high school (that's my excuse and I'm stickin' to it!)... LOL

 

[SimonM]

A quick helping guide that can be useful to know:

k + something = 1000 of it e.g 1k eggs = 1000 eggs; it is called kilo something

M = 1 000 000 (Mega)

m = 0.001 (mili)

u = 0.000 001 (micro)

n = 0.000 000 001 (nano)

p = 0.000 000 000 001 (pico)

Those suffixes are used frenquently in engineering of all sorts.

 

[pbandj]

I really do appreciate the help so if you don't mind I gotta a few more questions. You don't have to give me the answers cause i would rather just know it than have someone do it for me.

First one I just want to make sure I did it right. the question is "Find the current if 900 Coulombs pass a point in a circuit in 3 minutes" do you just take 900 and divide it by 3 and get 300A?

How would you find time? For instance the question is how long would it take to deposit 60C of charge if the current is 500mA?

Also how would you find energy? For instance how much energy is needed to move 8C of charge through a potential difference of 50V?

Again I appreciate it all the help and I would ask the teacher but he just is no help at all.

 

[MikeMl]

Amp= Coulomb/sec, not minute.

Coulomb=Amp*sec

Energy = (C*(V^2))/2

 

[pbandj]

Amp= Coulomb/sec, not minute.

Coulomb=Amp*sec

Energy = (C*(V^2))/2

oh duh...convert minutes into seconds. That helps! again thanks I must sound like an idoit or something!

 

[SimonM]

No your do not sound like an idiot We all have passed through the learning curve at one point. You even got the good spirit. Some people ask for help like this: Here is my homework...do it for me!

 

[MikeMl]

... Some people ask for help like this: Here is my homework...do it for me!

And if you noticed, I try to avoid just handing them the answer...

 

[pbandj]

ok another dumb question for ya. Here's what the problem says "What is the current if 63E16 electrons pass a point in a wire in 20ms." my question is how do you convert 63E16? I mean to find current you have to divide right? so I'm assuming its 63E16/20. unless I'm totally wrong here.

 

[dknguyen]

Close...

Amps = Coloumbs / Second
1 Coloumb = A certain number of electrons

I forget what that certain number is, but it's one of the constants, but I forget which one it is.

EDIT:
https://en.wikipedia.org/wiki/Coulomb

Remember that we are engineers not physicists so we distort things to our own ends (ie. conventional vs electron current flow...grrr...)

 

[pbandj]

thanks DK!

 

[Ratchit]

pbandj,

Here's what the problem says "What is the current if 63E16 electrons pass a point in a wire in 20ms." my question is how do you convert 63E16? I mean to find current you have to divide right? so I'm assuming its 63E16/20. unless I'm totally wrong here.

You won't get far if you are careless with your units as shown above.


(63E16 electrons/20 ms)*(1 coulomb/6.24151E18 electrons)*(1000 ms/1 sec) = 5.04 coulomb/sec = 5.04 amps

Ratch

 

[pbandj]

pbandj,




You won't get far if you are careless with your units as shown above.


(63E16 electrons/20 ms)*(1 coulomb/6.24151E18 electrons)*(1000 ms)/1 sec) = 5.04 coulomb/sec = 5.04 amps

Ratch

ok how did you get those numbers?

 

[Ratchit]

pbandj,

63E16 electrons/20 ms)----------->from your problem
1 coulomb/6.24151E18 electrons--->Coulomb - Wikipedia, the free encyclopedia
1000 ms)/1 sec------------------->1000 milliseconds = 1 second

All the units cancel out except coulombs and seconds.


Ratch

 

[pbandj]

Here's what I did. I took 63E16 divided it by 6.24151E18 and than divided that by 20ms could I do it that way? also I have similar problems but they want to know different things. such as what is the applied voltage and how long will it take to completely discharge the battery. so could I solve them doing it this way?


EDIT:

nevermind, gotta think OHMS LAW

 

[Ratchit]

pbandj,

Here's what I did. I took 63E16 divided it by 6.24151E18 and than divided that by 20ms could I do it that way?

Certainly. If you are straight in your mind about what you are doing. The method I showed gives you absolute assurance that you are doing the arithmetic and applying the units correctly.

I have similar problems but they want to know different things. such as what is the applied voltage and how long will it take to completely discharge the battery. so could I solve them doing it this way?

Yes. Show us the problem and your attempt to solve it.

How would you find time? For instance the question is how long would it take to deposit 60C of charge if the current is 500mA?

1 amp = 1 coulomb/1 sec =====> 1 sec = 1 coulomb/1 amp ====> 60 coulomb/0.5 amps = 120 coulomb/amps = 120 secs

Also how would you find energy? For instance how much energy is needed to move 8C of charge through a potential difference of 50V?

1 volt = 1 joule/1 coulomb ======>1 joule = 1 volt-coulomb ===> (50 volts)(8 coulomb) = 400 volt-coulomb = 400 joules


Ratch